1. Field of Invention
This invention relates to an improved switching power supply which saves power under a small load to a no load condition.
2. Description of the Prior Art
A conventional switching power supply is shown in FIG. 1 and comprises a transformer TR including a primary winding L1 and a secondary winding L2; a magnetic flux detector 10; a current loop circuit 40 for controlling a DC current applied to the primary winding L1; a feedback circuit 30 for detecting and feeding an output voltage supplied to a load Z, on the secondary winding L2 side, back to the current loop circuit 40; and a secondary circuit 20 for supplying a voltage from the secondary winding L2 to the load Z.
The magnetic flux detector 10 includes a first comparator TRCMP, wherein a magnetic flux energy accumulated in the transformer TR is detected as a voltage signal by means of a current I2 flowing through the secondary winding L2 and a resistor R1, and to which the voltage signal and a reference voltage Vt1 are applied; and a first flip flop circuit FF1, wherein the output signal V6 from the first comparator TRCMP is inputted to the set terminal S; a gate signal V2 for a switching device SW is inputted to the reset input terminal R; and an output signal V7 from the output terminal Q of the first flip flop circuit FFL is connected to the set input terminal S of the second flip flop circuit FF2, discussed hereinafter.
The secondary circuit 20 includes the secondary winding L2 of transformer TR; a rectifying diode D connected in series to the secondary winding L2; a capacitor C connected in parallel to the secondary winding L2; and a load Z connected in parallel to the capacitor C. The feedback circuit 30 is located on the output load side, where a voltage applied to the load Z and a reference voltage Vt2 are inputted to the feedback circuit 30 in order to negatively feed back a current control signal V4 from an error amplifier EA, which outputs the current control signal V4, to a second comparator CSCMP so that a given output voltage is maintained.
The current loop circuit 40 includes the second comparator CSCMP wherein a current L1 flowing through the primary winding L1 is detected by means of a resistor R2 and the resulting voltage signal V3 is inputted to the non-inverting input terminal and the current control signal V4 from the feedback circuit 30 is inputted to the inverting input terminal; the second flip flop circuit FF2 wherein the output signal V5 of the second comparator CSCMP is inputted to the reset input terminal R; the output signal V7 of the first flip flop circuit FF1 is inputted to the set input terminal S and the output signal V2 of the output terminal Q is inputted to the gate of the switching device SW; and the switching device SW is turned ON and OFF by means of the output signal V2 from the second flip flop circuit FF2. The switching device SW is connected in series to the primary winding L1 of the transformer TR to control the current L1 flowing through the primary winding L1.
The switching power supply of FIG. 1 is operated as follows, with reference to the timing chart of FIG. 2, wherein the current L1 (voltage signal V3) flowing through the switching device SW, for applying a voltage to the primary winding L1, reaches the current control signal V4; the output signal V5 of the second comparator CSCMP changes to a high state and the output V2 of the second flip flop circuit FF2 is changed from a high state to a low state to turn OFF the switching device SW. That is, the switching device SW remains turned ON until the current flowing through the switching device SW reaches the current control signal V4, so that magnetic flux energy is accumulated in transformer TR.
When the switching device SW is turned OFF, the magnetic flux energy accumulated in transformer TR is supplied to the load Z through the secondary winding L2, rectifying diode D, etc, as a load current.
When the magnetic flux energy in the transformer TR becomes depleted, as time lapses, the voltage of the secondary winding L2 drops rapidly to fall below the reference voltage Vt1. Thus, the output signal V6 of the first comparator TRCMP goes low and the output signal V7 of the first flip flop circuit FFL goes high, thereby causing the output signal V2 of the second flip flop circuit FF2 to be changed to a higher state. This in turn causes the switching device SW to be turned ON. If the switching device SW is turned ON, the current I1 flowing through the switching device SW (i.e. voltage signal V3) continues to rise until the current again reaches the current control signal V4 level. In this manner, the switching power supply repeats the above operation to sustain self excited oscillation.
This means that the self excited oscillation in the conventional switching power supply is based on the mechanism wherein energy accumulated in the transformer TR is controlled by switching ON and OFF, the switching device SW to the difference between the voltage detected on the secondary winding L2 side and the reference voltage Vt1
In the described switching power supply, the current supplied to the load is, in principle, inversely proportional to the oscillation frequency. This is because the energy exchanged with the transformer TR at each cycle is also reduced when the amount of current supplied to the load is decreased, thereby resulting in a shorter time interval at which the switching device SW is turned ON and OFF. When the ON-OFF time interval of the switching device SW is shortened, the frequency of self excited oscillation is increased. This could cause such problems as power loss in the switching device SW, core loss in the transformer TR, increase in noise, and failure in oscillation. An excess increase in the oscillation frequency thus must be avoided. For this purpose, the minimum load is fixed using a bleeder resistor in some cases. This could also cause a problem, namely, that the power consumption then is increased even when the load is small or there is no load at all, since increases in the above discussed losses and other losses result from the frequency increases. Consequently, in the convention apparatus, it is difficult to reduce power consumption.
Moreover, in the art, the unresolved problem is how to prevent the ON-OFF time interval of the switching device SW from becoming shortened more or less under a small load to no load condition, and thereby avoid any excess increase in the frequency of self-excited oscillation.
An unsatisfactory attempt to resolve the above problem is shown in FIG. 3, wherein attempt was made to prevent an increase in switching frequency by providing the switching power supply with an oscillator for outputting a fixed frequency pulse signal, that is high state and low state pulses, as ON-OFF signals, rather than using a magnetic flux detector 10 of FIG. 1. Further power saving is required, however, even when the switching power supply is under a small load condition or a no load condition.